A technique for forming a thin film transistor (TFT) with the use of a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been used in display devices typified by liquid crystal televisions. A silicon-based semiconductor material has been known as a semiconductor thin film applicable to a thin film transistor. As another material, an oxide semiconductor has attracted attention.
As an oxide semiconductor material, for example, zinc oxide and a substance containing zinc oxide have been known. In addition, a thin film transistor formed using an amorphous oxide (an oxide semiconductor) whose concentration of carriers (electrons) is lower than 1018/cm3 has been disclosed (References 1 to 3).